Coffin-Lowry syndrome: An X-linked form of mental retardation in which the affected males have short stature and characteristic face, finger and skeletal abnormalities. Facial features include prominent forehead, widespread eyes, downslanting eyeslits, prominent ears, thick lips and irregular/missing teeth. The hands are large and soft with lax skin and tapering fingers. Skeletal changes include kyphosis/scoliosis and pectus carinatum (pigeon breast) or pectus excavatum (caved-in chest). The syndrome is due to mutation of the gene on the X chromosome that encodes RSK2, a growth-factor regulated protein kinase.